This invention relates to an improved technique for adding an additional-information constituent to a transmitted NTSC television signal in a manner such that the additional information defined by this constituent may be recovered by, for example, a wide screen and/or extended-definition receiver without introducing any noticeable interference in the picture displayed by a standard NTSC receiver. The invention will be described in terms of adding higher frequency components of a video signal to a broadcast television signal, but it should be appreciated that it is applicable to adding practically any type of signal, e.g. enhanced sound signals, to the broadcast signal.
As is known, the video bandwidth of an NTSC television signal includes an upper sideband that extends to 4.2 MHz above the picture carrier frequency (with the sound carrier frequency being situated 4.5 MHz above the picture carrier frequency) and a vestigial lower sideband that extends only 1.25 MHz below the picture carrier frequency. Further, a standard NTSC receiver includes a so-called Nyquist filter that nominally passes 0% of that portion of the received television signal more than 0.75 MHz below the picture carrier frequency and 100% of that portion of the received television signal more than 0.75 MHz above the picture carrier frequency. In the frequency interval from 0.75 MHz below to 0.75 MHz above the picture carrier frequency, the nominal percentage of the received television signal that is passed rises linearly with respect to frequency, so that the percentage of the received television signal passed at the carrier frequency itself is 50%.
Quadrature amplitude modulation (QAM) is a known technique for adding the aforesaid additional-information constituent to a transmitted NTSC television signal without increasing the bandwidth of the signal. The problem with QAM is that the presence of the Nyquist filter in a standard NTSC television receiver has the effect of rendering the respective upper and lower sideband amplitudes of the QAM unequal, which results in interference being introduced into the picture displayed thereby. In this regard, reference is made to both the article "An Extended Definition Television System Using Quadrature Modulation of the Video Carrier with Inverse Nyquist Filter," Yasumoto et al., IEEE Transactions on Consumer Electronics, pp 173-180, (1987), and the article "An NTSC Compatible Wide Screen Television System with Evolutionary Extendibility," Kageyama et al., IEEE Transactions on Consumer Electronics, pp 460-468, (1988). These articles not only discuss in some detail this problem of interference introduced into the picture displayed in a standard NTSC television receiver by QAM, but suggest a solution therefor. The suggested solution involves passing the additional-information constituent, after it has double-sideband amplitude-modulated an IF or RF carrier which is in quadrature with the main picture carrier, through a filter which has an "inverse transfer function" with respect to the Nyquist filter employed by a standard NTSC television receiver, before the QAM signal is transmitted in combination with a regular NTSC television signal. It is understood that the Nyquist slope does not necessarily have to be straight as shown in FIG. 1, but must be conjugate and anti-symmetric around the carrier frequency to operate properly as a vestigial sideband filter in a TV receiver. If a transfer function is defined by a complex function H(fc+f) where fc is the carrier frequency and f is the frequency departure from the carrier frequency, then the inverse transfer function is defined here as a complex function H*(fc-f), i.e. the complex conjugate of H(fc+f) mirrored around the carrier frequency fc. The dashed line shown in FIG. 1 illustrates the amplitude response of the inverse Nyquist filter. The net transfer function for QAM is thus H(fc+f)H*(fc-f) which is conjugate symmetric around fc. As a consequence there is no crosstalk of QAM into NTSC with the result that at the output of the Nyquist filter of a standard NTSC television receiver, the in-phase component of the lower sideband of the QAM signal ideally is equal in amplitude but of opposite phase with respect to the in-phase component of the upper sideband of the QAM signal. Therefore, the respective in-phase components of the lower and upper sidebands of the QAM signal cancel one another in the detected NTSC video signal. Since the regular NTSC television signal modulates an in-phase carrier and the video detector of a standard NTSC television detects the in-phase modulation but responds very little, if at all, to quadrature modulation, interference in the detected video signal by the QAM signal due to the presence of the Nyquist filter of the receiver is eliminated, or at least substantially reduced, by employing an inverse Nyquist filter in the transmitter.
Because an inverse Nyquist filter, which functions as an attenuation equalizer, inherently has to have a different effect on the lower and upper sidebands of the QAM signal, it must operate at IF or RF frequencies. Such a filter is more costly and less stable than attenuation equalizers designed to operate at baseband frequencies. Further, in practice, such a filter is likely to require an additional frequency dependent delay correction in the RF domain in order to cancel the interference throughout the entire band of frequencies of the additional-information constituent.